Vacuum Ultraviolet Region Research Articles

Modulation of multiphoton resonant high-order harmonic spectra driven by two frequency-comb fields**Project supported by the National Natural Science Foundation of China (Grant Nos. 11504288, 11374239, 11534008, and 91536115) and the Fundamental Research Funds for the Central Universities, China.

The frequency-comb structure in the extreme ultraviolet (XUV) and vacuum ultraviolet (VUV) regions can be realized by the high-order harmonic generation (HHG) process driven by frequency-comb fields, providing an alternative approach for the measurement of an unknown frequency in XUV or VUV. We consider the case of two driving frequency-comb fields with the same repetition frequency and the carrier frequencies of fundamental- and third-harmonics, respectively. The many-mode Floquet theorem (MMFT) is employed to provide a nonperturbative and exact treatment of the interaction between a quantum system and the frequency-comb laser fields. Multiphoton transition paths involving both fundamental- and third-harmonic photons are opened due to the coupling of the third-harmonic frequency-comb field. The multiphoton transition paths are superpositioned when the carrier-envelope-phase shifts (CEPs) fulfill the matching condition. And the interference of the multiphoton transition paths can be controlled by tuning the relative envelope delay between the fields. We find that the quasienergy structure, as well as the multiphoton resonant high-order harmonic generation (HHG) spectra, driven by the two frequency-comb fields can be coherently controlled via the interference of multiphoton transition paths. It is also found that the spectral intensities of the generated harmonics can be modulated, and the modulation behavior is harmonic-sensitive.

12-Deoxyroridin J and 12-Deoxyepiisororidin E from Calcarisporium arbuscular

The culture broth of Calcarisporium arbuscular afforded 12-deoxyroridin J (1) and 12-deoxyepiisororidin E (2). Their structures were elucidated based on NMR and ECD analyses. Both molecules showed negative ECD splitting at 240 nm and a negative Cotton effect at 200 nm. Density functional theory calculations revealed that the former is caused due to the ECD exciton coupling between the C1′–C3′ enone and C7′–C11′ dienone chromophores, whereas the latter corresponds to the longer wavelength fragment of the negative ECD splitting in the vacuum ultraviolet (VUV) region at 190 nm which cannot be observed with conventional circular dichroism spectrometers. Simultaneous isolation of the regular roridins suggested that 1 and 2 are obtained through the same biosynthetic pathway. Since the 12-epoxy functionality is introduced at an early biosynthetic stage, 1 and 2 are assumed to be derived from reductions of the 12,13-epoxide in the corresponding regular roridins at a later biosynthetic stage.

On Precision Measurement of the Frequency of the 11S–23S Forbidden Transition of a Helium Atom

The possibility is demonstrated of a precision frequency measurement for the 11S0−23S1 forbidden transition (at 62.6 nm) of a helium atom using the stimulated Raman scattering method. The 11S0 singlet state is the ground state of 4He, while the metastable 23S1 state has the lowest energy in the triplet part of the spectrum (4He*). The transition has a very small natural width, which allows us to consider it as a possible reference for creation of a frequency standard in the vacuum ultraviolet region.

Photodissociation Dynamics of Nitrous Oxide near 145 nm: The O(1S0) and O(3P J=2,1,0) Product Channels.

We report the study of photodissociation dynamics of nitrous oxide in the vacuum ultraviolet region, using the time-sliced velocity map ion imaging technique. Ion images of the O(1S0) and O(3P J=2,1,0) products were measured at nine photolysis wavelengths from 142.55 to 148.79 nm. The product channels O(1S0) + N2(X1Σg+) and O(3P J=2,1,0) + N2(A3Σu+) have been observed. For these dissociation channels, the total kinetic energy releases of the dissociated products were acquired. With vibrational structures of the N2 coproducts partially resolved in the experimental images, the branching ratios of different vibrational states of the N2 coproducts were determined, and the vibrational state specific anisotropy parameters (β values) were derived. Analysis shows that the O(1S0) + N2(X1Σg+) channel is primarily formed via nonadiabatic couplings between the C (1Π) state and the higher-lying D (1Σ+) state of the N2O. A moderate rotational excitation and high vibrational excitation of N2(X1Σg+) products have been observed through this pathway. On the other hand, for the O(3P J=2,1,0) + N2(A3Σu+) channels, where a slightly higher rotational excitation of N2 coproducts have been observed, the possible pathway would be via nonadiabatic couplings from the C (1Π) state to the lower-lying A(1Σ-)state.

Acetylacetone photodynamics at a seeded free-electron laser

The first steps in photochemical processes, such as photosynthesis or animal vision, involve changes in electronic and geometric structure on extremely short time scales. Time-resolved photoelectron spectroscopy is a natural way to measure such changes, but has been hindered hitherto by limitations of available pulsed light sources in the vacuum-ultraviolet and soft X-ray spectral region, which have insufficient resolution in time and energy simultaneously. The unique combination of intensity, energy resolution, and femtosecond pulse duration of the FERMI-seeded free-electron laser can now provide exceptionally detailed information on photoexcitation–deexcitation and fragmentation in pump-probe experiments on the 50-femtosecond time scale. For the prototypical system acetylacetone we report here electron spectra measured as a function of time delay with enough spectral and time resolution to follow several photoexcited species through well-characterized individual steps, interpreted using state-of-the-art static and dynamics calculations. These results open the way for investigations of photochemical processes in unprecedented detail.

A Small Excimer Lamp for Photoionization Detectors

A small excimer glow-discharge argon lamp intended for use in portable gas-analytical photoionization devices is described. The emission spectrum of the lamp in the vacuum ultraviolet region and other characteristics necessary for its practical application are presented. The radiation flux emitted by the lamp in the range of 120−135 nm is approximately 1013 photons/s.

EUV polarimetry for thin film and surface characterization and EUV phase retarder reflector development.

The knowledge and the manipulation of light polarization state in the vacuum ultraviolet and extreme ultraviolet (EUV) spectral regions play a crucial role from materials science analysis to optical component improvements. In this paper, we present an EUV spectroscopic ellipsometer facility for polarimetry in the 90-160 nm spectral range. A single layer aluminum mirror to be used as a quarter wave retarder has been fully characterized by deriving the optical and structural properties from the amplitude component and phase difference δ measurements. The system can be suitable to investigate the properties of thin films and optical coatings and optics in the EUV region.

Radiation Characteristics of Air in the Ultraviolet and Vacuum Ultraviolet Regions of the Spectrum behind the Front of Strong Shock Waves

Experiments on the measurement of air emission intensity behind the front of incident shock wave were carried out in a shock tube at an initial pressure of 0.25 Torr and shock wave velocities of 6.3–8.4 km/s. The emission intensity was measured in absolute units both in the form of an integral spectral distribution in a wavelength range of 120−400 nm (panoramic spectra) and as the time evolution of emission at the individual atomic lines of nitrogen and oxygen atoms. The results of the measurements demonstrated that the emission in air behind a shock wave in the vacuum ultraviolet region of 120–200 nm had a much higher radiation flux level than the emission in a range of 200–900 nm.

Spectroscopy of N,N-dimethylformamide in the VUV and IR regions: Experimental and computational studies.

The electronic absorption spectrum of N,N-dimethylformamide (DMF) is studied in the 45 000-80 000 cm-1 (5.6-9.9 eV) region using synchrotron radiation. The vacuum ultraviolet (VUV) spectrum comprises mostly of Rydberg series of ns, np, and nd types converging to the first two ionization potentials (IPs). Quantum defect values obtained are consistent with excitation of an electron from the highest occupied molecular orbitals localized on nitrogen (4a″) and oxygen (16a'); in addition, the 3s Rydberg transition converging to the third IP (3a″) is observed at 8.95 eV. A reinvestigation of the infrared spectrum of DMF in the 500-4000 cm-1 region with the help of density functional theory (DFT) calculations establishes the planarity of the ground state and leads to revision of several vibrational assignments. Vertical excited state energies and their valence/Rydberg character are predicted using time dependent DFT calculations; excellent correlation is achieved between theoretical results and experimentally observed spectral features. Potential energy curves of the first few excited states give additional insights into the nature of the excited states and their role in photodissociation dynamics. The absorption spectrum of DMF in the region >63 400 cm-1 (7.85 eV) as well as a complete set of spectral assignments in the VUV region (45 000-80 000 cm-1) is reported for the first time. This work represents a comprehensive study of the absorption spectra of DMF in the VUV and infrared regions.

Luminescence properties of organic–inorganic layered perovskite-type compounds under vacuum ultraviolet irradiation

We investigated the luminescence properties of organic–inorganic layered perovskite-type compounds under vacuum ultraviolet irradiation. A crystal of (C6H5C2H4NH3)2PbBr4 was fabricated by the poor-solvent diffusion method. Exciton emissions from the inorganic layer were observed at 410 nm under ultraviolet irradiation (excitation wavelengths: 180 and 300 nm). The rise time behavior observed in the luminescence decay curve showed no difference among the excitation wavelengths of 60–300 nm. In addition, no excitation peak of benzene such as an intense peak at 180 nm (1A1g → 1E1u) in the vacuum ultraviolet region was observed in the excitation spectra measured while monitoring the exciton emissions from the inorganic layer. These results indicate that the effect of energy transfer from the organic layer to the inorganic layer has negligible contribution to the luminescence properties of organic–inorganic layered perovskite-type compounds.

Detection of Vacuum Ultraviolet light by means of SiPM for High Energy Physics experiments

Silicon Photo-Multipliers (SiPMs) are now widely used as light detectors for several high energy physics experiments. The performance of these devices for the detection of scintillation light of liquefied noble gases, such as Liquid Xenon (LXe) or Liquid Argon (LAr), that emit photons in the Vacuum Ultra-Violet region (VUV, λ=100÷200 nm) will be presented. In particular, SiPM arrays produced by Hamamatsu, SensL and AdvanSiD have been tested in terms of quantum efficiency using a VUV measurement system based on a monochromator, a deuterium lamp and a NIST calibrated photodiode. A comparison between standard (plastic window) and completely naked devices has been carried out on the wavelength range of λ=120÷170 nm.Sensitivity in detecting VUV light of tested devices is comparable to standard bialkali PMTs covered with wavelength shifter.

Results of Spectral Corona Observations in Solar Activity Cycles 17–24

The results of the work of the global observation network are considered, and a comparative analysis of the data of various coronal observatories is performed. The coronal activity index has been reconstructed for the period 1939–2016 based on the data of various observatories in Kislovodsk system. For this purpose, the corona daily intensity maps from the Sacramento Peak and Lomnický Stit observatories according to the Solar-Geophysical Data journal have been digitized; they supplement the data of other observatories. The homogeneity and continuity of the corona observations at the Kislovodsk station, including activity cycle 24, is confirmed. Unfortunately, the only observatory at present that continues observation of the spectral corona in Fe XIV 5303 A and Fe XIV 6374 A lines is the Kislovodsk astronomical station Mountain Astronomical Station (MAS) of the Central Astronomical Observatory, Russian Academy of Sciences (Pulkovo). The data on the combined corona in 5303 A line are analyzed. It is shown that there is a high correlation of the intensity index of green corona with solar radiation measurements in the vacuum UV region. Data on the beginning of the new 25th activity cycle in the corona at high latitudes are presented.

Photofragmentation of gas-phase acetic acid and acetamide clusters in the vacuum ultraviolet region.

Photofragmentation of gas-phase acetamide and acetic acid clusters produced by a supersonic expansion source has been studied using time-of-flight mass spectrometry and the partial ion yield (PIY) technique combined with tunable vacuum-ultraviolet synchrotron radiation. Appearance energies of the clusters and their fragments were experimentally determined from the PIY measurements. The effect of clusterization conditions on the formation and fragmentation of acetic acid clusters was investigated. Ab initio quantum mechanical calculations were performed on both samples' dimers to find their neutral and ionized geometries as well as proton transfer energy barriers leading to the optimal geometries. In the case of the acetamide dimer, the reaction resulting in the production of ammoniated acetamide was probed, and the geometry of the obtained ion was calculated.

Realization of a high vacuum evaporation system for wave-length shifter deposition on photo-detector windows

In this paper, the authors present a system based on a vacuum chamber and Knudsen cells that can evaporate organic materials on convex surfaces up to 10 in. diameter, with a good uniformity, in a very short time. This system was successfully used at CERN for the deposition of tetra-phenyl butadiene wavelength shifter on the 8 in. window of 360 Hamamatsu R5912-MOD photomultiplier tubes (PMTs), before their installation inside the ICARUS T600 detector to be used for neutrino physics studies. The quantum efficiency in the vacuum ultraviolet region measured on the whole photocathode shows a good uniformity over the entire PMT active surface with an average value above 10% from 120 to 200 nm.

Optical Spectroscopy of Ca9Tb1–xEux(PO4)7 (x = 0, 0.1, 1): Weak Donor Energy Migration in the Whitlockite Structure

The luminescence spectroscopy of Tb3+ and Eu3+ has been studied in the Ca9Tb(PO4)7, Ca9Eu(PO4)7, and Ca9Tb0.9Eu0.1(PO4)7 materials having a whitlockite structure, by using excitation in the near UV, vacuum UV and X-ray regions. The Eu3+ ion in Ca9Eu(PO4)7 is located mainly in two cationic sites, as evidenced by the fine structure of the 5D0 → 7F0 transition at 5 K. In the case of Ca9Tb0.9Eu0.1(PO4)7, weak Tb3+ → Eu3+ energy transfer is observed upon excitation in the UV bands of Tb3+. The low efficiency of the transfer appears to be due to slow energy migration in the 5D4 subset of the Tb3+ ions. The overall behavior is strongly affected by the multisite and disordered nature of the Tb-based whitlockite host.

VUV-UV photoluminescence properties of Ce3+ doped Ca2SnO4 phosphor for PDP application

Calcium stannate oxide (Ca2SnO4) compound doped with Ce3+ rare earth ion was synthesized by conventional solid state synthesis method at 1300°C. Phase confirmation and crystal structure analysis was done by obtaining its X-ray diffraction (XRD) pattern. The vacuum ultraviolet (VUV) – visible spectroscopic properties were investigated. Ca2SnO4:Ce3+ phosphor possesses broad and strong absorption in VUV and UV region. Excitation peaks are present at 140nm, 174nm, 200nm and 261nm. Ca2SnO4:Ce3+ exhibits intensive photoluminescence (PL) emission centered at 430nm and 467nm under VUV excitation. CIE chromaticity co-ordinates were calculated. The luminescence properties of Ca2SnO4:Ce3+ compound make it good candidate as a blue phosphor for mercury-free lamps and plasma display panel (PDP) applications.

Visible quantum cutting in green-emitting BaF2: Gd3+, Tb3+ phosphor: An approach toward mercury-free lamps

Visible quantum cutting (QC) has been observed in green emitting BaF2 co-doped with Gd3+, Tb3+ phosphor via down-conversion (DC) synthesized by wet chemical method. Powder X-ray diffraction (XRD) analysis shows structural purity of synthesized phosphors. The excitation (PLE) and PL spectra in the vacuum ultraviolet (VUV) or UV region were measured with the help of 4B8-VUV spectroscopy experimental station of the Beijing Synchrotron Radiation Facility (BSRF). In the QC process, one VUV–UV photon absorbed is cut into more than one visible photons emitted by Tb3+ through cross relaxation (CR) and direct energy transfer (DET) between Tb3+ and Tb3+ or Tb3+ and Gd3+, depending on the excitation wavelength. From the emission spectra monitored at different wavelength excitation, the two-step energy transfer process is investigated and theoretically calculated quantum efficiency observed 148% and 177% at the excitation wavelength of 174 nm and 219 nm respectively.

Ultraviolet discharges from a radio-frequency system for potential biological/chemical applications

In this work, we describe a new electrode-less radio-frequency (RF) excitation technique for generating excimers in the vacuum ultraviolet (VUV) and ultraviolet (UV) spectral regions for potential biological/chemical applications. Spectra data of Xe, XeI, and KrI generated by this new technique are presented. Optical efficiency of the lamp system ranges from 3% to 6% for KrI, 7% to 13% for XeI, and 15% to 20% for Xe. Also, results of irradiating E-coli with XeI discharge from this lamp system is presented to show one of the promising applications of such electrode-less apparatus. This new RF lamp system offers an interesting addition to the already existing technologies for generating VUV and UV light for various biological, physical, and chemical processes especially those requiring large area for high productivity.

Scintillation and dosimeter properties of CaF2 transparent ceramics doped with Nd3+ produced by SPS

We have developed CaF2:Nd transparent ceramics with varying doping concentrations of Nd by spark plasma sintering (SPS) and evaluated the optical, scintillation and dosimeter properties. The samples showed effective absorption peaks in the visible and near infrared regions due to the 4f-4f transitions of Nd3+. In scintillation properties, Nd3+ was also active and showed the 4f-4f radiative transitions of Nd3+ which appeared at 860 and 1064nm under X-ray irradiation. In photoluminescence under 160nm excitation, the samples showed emission peaks in the vacuum ultraviolet region due to the 5d-4f transitions of Nd3+. Furthermore, the samples showed thermally stimulated luminescence (TSL) exhibiting glow peaks at 100, 150 and 380°C. Among the present samples the 5% Nd-doped sample showed the highest sensitivity which allowed to measure radiation dose from 0.1 to 1000mGy with a good linear response.

Formation of H a - hydrogen centers upon additive coloration of alkaline-earth fluoride crystals

The mechanism of coloration of alkaline-earth fluoride crystals CaF2, SrF2, and BaF2 in calcium vapors in an autoclave with a cold zone is studied. It was found that the pressure in the autoclave upon constant evacuation by a vacuum pump within the temperature range of 500–800°C increases due to evaporation of metal calcium. In addition to the optical-absorption bands of color centers in the additively colored undoped crystals or to the bands of divalent ions in the crystals doped with rare-earth Sm, Yb, and Tm elements, there appear intense bands in the vacuum ultraviolet region at 7.7, 7.0, and 6.025 eV in CaF2, SrF2, and BaF2, respectively. These bands belong to the Ha - hydrogen centers. The formation of hydrogen centers is also confirmed by the appearance of the EPR signal of interstitial hydrogen atoms after X-ray irradiation of the additively colored crystals. Grinding of the outer edges of the colored crystals leads to a decrease in the hydrogen absorption-band intensity with depth to complete disappearance. The rate of hydrogen penetration inside the crystal is lower than the corresponding rate of color centers (anion vacancies) by a factor of tens. The visible color density of the outer regions of the hydrogen-containing crystals is several times lower than that of the inner region due to the competition between the color centers and hydrogen centers.